Coarse-grained pipe structures as indicators of active seismicity in the Gondwana foreland, western Argentina.

EZPELETA M. Y ASTINI, R.A.

Banff, Canadá.

Congreso; Alluvial Fans Meeting 2007; 2007

Large-sized (m’s-10 m’s) vertical structures record unusual fluidization processes in the Late Paleozoic Paganzo foreland basin presently exposed in the Famatina Ranges. Similar giant structures have been interpreted as a product of seismicity and frequently involved silt-mudstones deltaic sequences (e.g. Prior and Coleman, 1982) or deeply buried remobilized eolian sands (e.g. Huuse et al., 2005). However, there are few records of seismicity and large-sized disruptions associated to alluvial fan sequences. In the Agua Colorada Formation (Upper Pennsylvanian) coarse-grained pipes are as wide as 30 m and have exposed heights ~40 m, involving mainly conglomeratic and coarse-sandstones facies characteristic of proglacial middle to distal alluvial fans. This interval contains highly disrupted and plastically deformed (soft sediment folds) zones with vertical pipes that have inverted cone geometry, that intrude and partially cross-cut an overlying lacustrine succession. Internally these giant conglomerate pipes have a broad subvertical lamination deformed into multiple contortions toward the margins. The maximum clast size included in these soft sedimentary structures is 0,3 m including granitic clasts and broken pieces of sidewall material (consolidated sandstones). In the De la Cuesta Formation (Lower Permian), a distinct coarse-grained relatively thick interval (3-4 m) interbedded in a distal alluvial-fan environment develops various deformation features including discrete cylindrical pipe-like structures and contorted lamination. The interval is formed by fine sandyconglomerates and coarse sandstones with little inherited primary structures. The maximum clast size is 0,08 m. Individual pipes are recognized throughout the interval and develop normal gradation from borders to centers. Abundant interstitial silt and chaotic to upward-finning textures seem to develop. Pipes are separated by 5-6 m of less perturbed zones. The deformed interval is abruptly truncated by an erosion surface on top of which a fine-grained thin-bedded largely tabular succession rests. The overlying succession contains graded beds, rippled tops and fallout mudstones and is interpreted as a shallow lacustrine facies association. Vertical burrow within the lacustrine beds are undisrupted. Regardless their scales, the deformed intervals in both units are interpreted as vent structures indicating escape of fluids through pipes. These conduits show dragging toward the surface (vertical lamination and gradation) together with chaotic fabrics indicating strong liquefaction and fluidization processes affecting saturated material. Obermeier et al. (2002) characterizes liquefaction as a process that occurs at relatively shallow depths (mainly between 1 and 5 meters), triggered by seismic activity. This phenomenon occurs abruptly in unconsolidated and water-saturated sediments and indicates sudden despressurization. Our pipes are interpreted as vent structures intimately related to surface coarse-grained volcanoes, which unfortunately have little preservation and have been truncated by erosion, particularly in the example of the De La Cuesta Formation. Rodríguez Pascua et al. (2000) interpreted that similar structures, but smaller (intruded and fractured gravels), were created by earthquake magnitudes >Mw 7,5. Although other indicators like megaboulder beds may be interpreted as seismites within the late Paleozoic synorogenic strata, by analogy with other megabeds elsewhere, pipe features are strong evidence in support of seismic shocks. According to available work (e.g., Atkinson, 1984) scaling and involved grain size allow indirect determination of the seismic shock magnitude. However, we predict that thickness and porosity-permeability of the fluidized interval, as well as that of the overlying strata and burial compaction, should be critical to the development of this kind of features. The particular megapipes preserved in the Agua Colorada Formation are a good case for paleoenvironmental control wherein a postglacial lacustrine interval operated as a magnificent seal for proglacial coarse-grained deposits triggering massive fluidization of sandy conglomerates.